Transmission for driving counter-rotating propellers, lubrication system, and associated methods

ABSTRACT

A hybrid gear/sprocket-based transmission is provided for driving a pair of coaxial, counter-rotating propellers in aircraft and airboats. A drive shaft couplable to an engine crank shaft extends rearward into the transmission case, and a pair of coaxial hollow driven shafts extend rearward out of the transmission case, to which are attachable a pair of propellers. A gear train, containing an even number of gears, reverses the rotational direction of the engine; a sprocket train, containing two sprockets and a chain connecting them, retains the rotational direction of the engine. Improved stability characteristics are imparted by supporting the drive shaft at two points within the case and also by positioning the drive and the driven shafts in vertical alignment. The adaptability of the transmission to being coupled with an automobile engine confers improved noise, reliability, and fuel efficiency characteristics.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from provisional application60/177,051, “Transmission for Driving Counter-Rotating Propellers,Lubrication System, and Associated Methods,” filed Jan. 20, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to propulsion mechanisms forvessels driven by propellers, and, more particularly, to transmissionsfor aircraft and airboats, and to lubrication systems therefor.

[0004] 2. Description of Related Art

[0005] Transmissions for driving a pair of counter-rotating propellersare known in the art for both airplanes and airboats. Such transmissionsare known to be driven by belts and by gears, with input typicallyprovided by a shaft driven by an aircraft engine operating atapproximately 2500-3000 revolutions per minute (rpm). Aircraft enginesare extremely expensive, noisy, and fuel inefficient.

[0006] Airboat systems that utilize belt-driven transmissions areinefficient owing to power losses caused by belt friction, especially athigher rotational velocities. Belt breakage in these systems is a sourceof failure. Another disadvantage of belt-driven systems is theirinability to permit reduction of engine speed, since the shaft used toeffect such a reduction would have to be too small to be practicable.Thus it has been deemed advantageous to utilize a different transmissionmethod in an airboat to enable engine speed reduction without loss ofefficiency.

[0007] A belt-driven, two-engine counter-rotating system has beendisclosed by Stewart (U.S. Pat. No. 4,426,049). It has been taught byBecker et al. (U.S. Pat. No. 4,932,280) to use coaxial drive shaftsystems for driving multiple outputs from a single input in an aircraft.Gearing means are disclosed for driving two outputs at different speeds.

[0008] A double-sprocket and wheel transmission for driving multiplepropellers in the same direction is described by Fay (U.S. Pat. No.1,329,387).

[0009] The use of a gear-based transmission for airboats has been taughtby Kaye (U.S. Pat. No. 5,807,149), including a transmission for drivinga pair of counter-rotating coaxial shafts, to each of which is affixed apropeller. Such an arrangement can be used with an automobile engine,which is far more economical than an aircraft engine. This transmissionhas been shown to reduce noise and torque, to permit varying gearratios, to increase fuel efficiency and engine life, and to be lessexpensive to operate.

[0010] Improved gear-based transmissions for airboats have also beendisclosed by Jordan (U.S. Pat. Nos. 5,724,867 and 6,xxx,xxx, the entirecontents of both of which are incorporated herein by reference). In the'xxx patent a lubrication system is also taught that includes a gear fordriving lubricant from a well to the interior of the inner output shaft,out of that space to a pair of stiffener bearings, and into the spacebetween the output shafts.

[0011] Another source of failure in transmissions is failure of the oilpump that heretofore has been considered an essential element.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide atransmission that has improved strength and stability characteristicsfor driving a pair of counter-rotating propellers.

[0013] It is a further object to provide such a transmission having asingle input shaft for driving means for driving the two output drivenshafts.

[0014] It is another object to provide a transmission for drivingcoaxial counter-rotating propellers that is drivable at variable orequal speeds as desired.

[0015] It is an additional object to provide such a transmission withwhich it is possible to drive the counter-rotating propellers atdifferent speeds to provide additional thrust, reduce noise output, andimprove fuel efficiency.

[0016] It is also an object to provide such a transmission that isconsiderably lighter than previously known devices.

[0017] A further object is to provide a system and method forlubricating the output driven shafts that reduces weight and complexity.

[0018] Another object is to provide a system and method for achieving asimplified lubrication of a transmission and propulsion for an airboat.

[0019] An additional object is to provide such a system and method thateliminates the need for an oil pump to circulate lubricant.

[0020] These and other objects are achieved by the transmission of thepresent invention, which is for driving a pair of coaxial,counter-rotating propellers for, for example, an airboat or an aircraft,although these are not intended as limiting usages. The transmission ishoused in a case that has an interior space.

[0021] A drive shaft extends from the outside of the case into theinterior space and is rotatable in a first direction. When thetransmission is in use on an aircraft or airboat, the drive shaft ismated at one end to a motor crank extending from and rotated by anengine. As mentioned above, previously known aircraft and airboats haveutilized aircraft-type engines; however, with the transmission of thepresent invention, it has been found that an automobile engine can beused. This has a benefit of reducing cost, as well as other benefits tobe discussed below.

[0022] A first driven shaft also extends into the interior space of thecase, typically from a side opposite that of the drive shaft. The firstdriven shaft is for rotating an outer propeller, that is, the propellerfarther from the airboat body.

[0023] A second driven shaft, which is hollow, likewise extends into theinterior space of the case and is further positioned in surrounding,generally coaxial arrangement to the first driven shaft. The seconddriven shaft is shorter than the first, and both ends protrude beyondthe ends of the first driven shaft. This second driven shaft is forrotating an inner propeller, that is, the propeller closer to theairboat body.

[0024] A gear train for driving the first shaft is housed in theinterior space of the case. In its simplest configuration, the geartrain comprises two gears: a drive gear and a driven gear. The drivegear is coaxially affixed to the drive shaft. The driven gear iscoaxially affixed to the first shaft in such a position and configuredso as to be rotatable by the drive gear. Thus, when the drive shaftrotates in the first direction, the drive gear is rotated in the firstdirection. This causes the first driven gear to be rotated in a seconddirection opposite in sense to the first direction, which consequentlydrives the first shaft in the second direction.

[0025] In an alternate embodiment, additional intermediate driven gearsmay be interposed between the drive gear and the first driven gear, solong as the total number of intermediate gears is an even number.

[0026] A sprocket train is also housed in the interior space of thecase. This sprocket train includes a first and a second sprocket and achain. The first sprocket is coaxially affixed to the drive shaft. Thesecond sprocket is coaxially affixed to the first shaft. The chain is inengagement with both sprockets and is in such a position and configuredso as to rotate the second sprocket upon the first sprocket turning.Thus, when the drive shaft rotates in the first direction, the firstsprocket is rotated in the first direction. This causes the secondsprocket also to be rotated in the first direction, which consequentlydrives the first shaft in the first direction.

[0027] Thus it can be seen that the rotation of the drive shaft in onedirection achieves, through the action of the hybrid transmissioncomprising the gear train and the sprocket train, a counter-rotation ofthe two coaxial driven shafts and thus imparts counter-rotation topropellers attached thereto. There is no known system that uses fewercomponents than that of the present invention, which permits lowerweight, improved efficiency, and enhanced reliability.

[0028] Using the present hybrid gear/sprocket transmission permitsdriving an automobile engine at the point of maximum horsepower, whichgenerally implies a motor crank rotational speed approximately in therange of 5000-5200 rpm, and then gearing down the rotational speed toroughly 1200-2800, possibly even lower, a quieter speed at which to runthe propellers.

[0029] The invention is not, of course, limited to the use of anautomobile engine; in fact, the presence of the gear and sprocket trainsenables the user to optimize for efficiency and noise characteristics byaltering gear ratios as desired. An aircraft engine may also be used.

[0030] Yet another feature of the present invention is an improvedlubrication system, in which an oil pump is no longer necessary, owingto the presence of the two coaxial driven shafts, between whichlubricant may pass and be moved by the counter-rotation thereof. Thisfeature of the system comprises means for injecting a lubricant into aspace between the driven shafts and means for blocking the entry holeduring use to retain the lubricant therein.

[0031] Among the benefits of the present invention are a minimization ofcomponents, which is believed to increase reliability and dependability,and a decrease in the weight, which increases performance and fuelefficiency. Present counter-rotator transmissions known in the art havea weight of 140-200 lbs, whereas the hybrid transmission of the presentinvention has a weight in the range of 90-110 lbs.

[0032] The features that characterize the invention, both as toorganization and method of operation, together with further objects andadvantages thereof, will be better understood from the followingdescription used in conjunction with the accompanying drawing. It is tobe expressly understood that the drawing is for the purpose ofillustration and description and is not intended as a definition of thelimits of the invention. These and other objects attained, andadvantages offered, by the present invention will become more fullyapparent as the description that now follows is read in conjunction withthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a side cross-sectional view of a first embodiment of atransmission for driving counter-rotating propellers, also showing theshaft lubrication system.

[0034]FIG. 2 is a side cross-sectional view of a second embodiment of atransmission for driving counter-rotating propellers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] A description of the preferred embodiments of the presentinvention will now be presented with reference to FIGS. 1 and 2.

[0036] The hybrid transmission 10 of the present invention, shown fromthe side in FIG. 1, which is designed to drive a pair of coaxial,counter-rotating propellers 20 and 30, comprises a case 50 that has aninterior space 502, a fore side 504, and an aft side 506. It ispreferred that the case exterior be aerodynamically shaped in order toconfer good airflow characteristics to the transmission 10 during use athigh speeds.

[0037] A drive shaft 12 extends into the interior space 502 of the case50 through the case's fore side 504. The drive shaft 12 is rotatable ina first direction, shown here as counterclockwise when viewed from thefront. Typically the drive shaft's fore portion 122, which extendsoutside the case 50, contains a coupler 124 for mating with a crankshaft 62 from an engine 60, which generates the rotational motion. Thedrive shaft 12 is preferably configured as a through shaft with respectto the case 50, and is thus supportable via brackets 508,509 affixed onthe inside of both the fore 504 and the aft 506 sides, respectively, ofthe case's interior space 502. This dual support confers exceptionalstability to the drive shaft 12.

[0038] The outer propeller 20 is mounted via propeller mount 202adjacent the aft end 224 of, and is rotated by, a first driven shaft 22that extends from the back side 506 into the interior space 502 of thecase 50. The fore end 222 of the outer propeller 20 is supported viabracketing 510 on the inside of the case's fore side 504. Preferably thefirst driven shaft 22 comprises a hollow shaft, having a lumen 221therethrough. A removable cap 223 at the aft end 224 leads to the lumen221, as does a hole 225 adjacent the fore end 222.

[0039] The inner propeller 30 is mounted via propeller mount 302 to theaft portion 324 of, and is rotated by, a second hollow driven shaft 32that extends from the aft side 506 into the interior space 502 of thecase 50. The second hollow shaft 32 is positioned in surrounding,generally coaxial arrangement to the first hollow driven shaft 20 and isshorter than the first driven shaft 22. These relative lengths permitthe fore end 222 and the aft portion 224 of the first driven shaft 22 toprotrude, respectively, beyond the fore end 322 and the aft portion 324of the second driven shaft 32. The second driven shaft 32 is supportedon the interior of the case's aft side 506 by bracketing 511.

[0040] In a preferred embodiment the longitudinal axes of the driveshaft 12 and the first 22 and second 32 hollow driven shafts arepositioned generally in vertical alignment. This positioning confersimproved stability during use, as the gyroscopic forces balanceoptimally in this configuration.

[0041] The transmission 10 of the present invention further comprisestwo drive trains housed within the case 50, one for driving each of thehollow driven shafts 22,32. The first train 40 comprises a gear traincomprising an even number of gears for changing the incoming rotationaldirection. The embodiment shown in FIG. 1 contains two gears: a drivegear 402 coaxially affixed to the drive shaft 12 and a driven gear 404coaxially affixed to the first hollow shaft 22. The driven gear 404 ispositioned and configured so as to be rotatable by the drive gear 402.Thus, when the drive shaft 12 rotates in the first direction, here shownas counterclockwise, the drive gear 402 is rotated in the samedirection, and the driven gear 404 is rotated in a second directionopposite in sense to the first direction, that is, clockwise. Thus thefirst hollow shaft 22 is driven in a clockwise direction also, as wouldbe an attached propeller 20.

[0042] The second drive train 42 comprises a sprocket train formaintaining the incoming rotational direction. The sprocket train 42comprises a drive sprocket 422, a driven sprocket 424, and a chain 426.The drive sprocket 422 is coaxially affixed to the drive shaft 12, andthe driven sprocket 424 is coaxially affixed to the second hollow shaft32. The chain 426 is positioned in encompassing relation to the teeth ofthe sprockets 422,424 and is configured so as to rotate the drivensprocket 424 upon a rotation of the drive sprocket 422. Therefore, inuse, when the drive shaft 12 rotates in the first direction, the drivesprocket 422 is rotated in the first direction, the driven sprocket 424is also rotated in the first direction, and the second hollow shaft 32is rotated in the first direction, conferring counter-rotationalmovement to the inner propeller 30 with respect to the outer propeller

[0043] In an alternate embodiment 10′ (FIG. 2), the gear train 40 andthe sprocket train 50 are interchanged in axial position, with the geartrain 40 driving the second hollow driven shaft 22 and the sprockettrain 50 driving the first hollow driven shaft 32.

[0044] Particular benefits of the hybrid propulsion system of thepresent invention include the possibility of using an even number ofgears, since a planetary, intermediate, sense-changing gear is no longernecessary to achieve counter-rotation. In addition to the weight andcommensurate efficiency advantage conferred thereby, stability is alsoimproved, with balancing force vectors conferring added reliability anddurability. For example, failures are known to have been caused byoutward-pointing forces imposed upon the planetary gear by the drivingand driven gears. Further, all the gears in the present invention aremounted on through shafts, which are also stronger and more stable.

[0045] In either of the above-detailed embodiments it may be seen thatthe first and the second drive trains can be adapted to drive thepropellers at different speeds, if desired which can provide improvedthrust characteristics, increased fuel efficiency, and reduced noise.

Shaft Lubrication System

[0046] An additional aspect of the present invention comprises alubrication system for delivering lubricant to elements of thepropulsion system. A particular embodiment of the lubrication system,illustrated in FIG. 1, comprises means for injecting, sealing, andcirculating a lubricant within an enclosed space including the hollowshafts 22,32 without the use of a mechanical pump. Upon removal of thecap 223, lubricant may be injected into the inner shaft's lumen 221. Therotational motion of the shaft 22 drives lubricant via “centrifugalforce” from the inner shaft's lumen 221 out through a plurality of holes228 in the aft portion of the shaft 22 to enter the space between theshafts 22,32, where there are positioned a plurality of floatingcylindrical bearings 66, which maintain the distance between the shafts22,32 and also assist to distribute lubricant. In a preferred embodimentthere are between two and four of these bearings 66 positioned in spacedrelation from each other between the shafts 22,32, and the materialcomprises brass. Although brass is disclosed herein, it will beunderstood by one of skill in the art that another material may be used,preferably a metal dissimilar from the material of which the shafts22,32 are composed.

[0047] In order to provide a path for the escape of trapped air in thespace to be lubricated, a toroidal collar 44 is provided that ispositioned around the inner shaft 22 between the gear 404 and thebearing 510. To an upper end of the collar 44 is affixed a line 45 thatleads at a top end from the case's 50 top side 512, and is sealable witha removable cap 46. The line 45 leads at a top end to a void 441 in thecollar 44 that surrounds the inner shaft 22. Two O-rings 47 arepositioned in surrounding relation to the collar 44 around the innershaft 22 to prevent lubricant from leaking out from the void 441 to theexterior of the inner shaft 22. Thus, in use, lubricant that has beeninjected into the inner shaft's lumen 221 also proceeds forward throughthe lumen 221 and reaches the collar's void 441. Any air bubbles thathave been trapped along the way can then escape through the uncappedline 45, and then the cap 46 can be replaced. The collar 44 does notturn with the shaft 22, but rather floats thereon, being heldsubstantially in place by the line 45.

[0048] The bearings 66 themselves represent a novel lubrication element,being designed to maximize lubricant return in the fore direction. In apreferred embodiment each bearing 66 has a series of generally helicalgrooves 662 cut in the outer surface, through which the lubricant maymove to be distributed within the outer shaft 32.

[0049] This lubrication system eliminates the oil pump and associatedgear previously known and used in the art, and thus also improves fuelefficiency by reducing weight by 15-20 lbs.

[0050] It may be appreciated by one skilled in the art that additionalembodiments may be contemplated, including variable numbers and sizes ofgears, which may be positioned and configured to permit variablerelative speeds of the two counter-rotating propellers.

[0051] In the foregoing description, certain terms have been used forbrevity, clarity, and understanding, but no unnecessary limitations areto be implied therefrom beyond the requirements of the prior art,because such words are used for description purposes herein and areintended to be broadly construed. Moreover, the embodiments of theapparatus illustrated and described herein are by way of example, andthe scope of the invention is not limited to the exact details ofconstruction.

[0052] Having now described the invention, the construction, theoperation and use of preferred embodiment thereof, and the advantageousnew and useful results obtained thereby, the new and usefulconstructions, and reasonable mechanical equivalents thereof obvious tothose skilled in the art, are set forth in the appended claims.

What is claimed is:
 1. A transmission for converting an engine driveshaft rotation into coaxial counter-rotations for driving a pair ofpropellers, the transmission comprising: fore driven means affixable toan inner shaft for rotating an outer propeller; aft driven meansaffixable to a hollow outer shaft coaxial with the inner shaft forrotating an inner propeller, the aft driven means coaxial with the foredriven means; a drive shaft having an aft end rotatably affixable withinthe housing and a fore end having means for being rotatably driven; foredrive means affixed for corotation with the drive shaft and having meansfor driving the fore driven means in a first direction; and aft drivemeans affixed for corotation with the drive shaft and having means fordriving the aft driven means in a second direction opposite the firstdirection.
 2. The transmission recited in claim 1, wherein: the foredriven means comprises a driven gear having teeth, the fore drive meanscomprises a gear, and the fore drive means driving means comprises gearteeth positioned for meshing with the driven gear teeth; and the aftdriven means comprises a driven sprocket having teeth, the aft drivemeans comprises a drive sprocket, and the aft drive means driving meanscomprise teeth; and the transmission further comprises a chain meshablewith the teeth of the driven sprocket and the drive sprocket.
 3. Thetransmission recited in claim 1, wherein: the aft driven means comprisesa driven gear having teeth, the aft drive means comprises a gear, andthe aft drive means driving means comprises gear teeth positioned formeshing with the driven gear teeth; and the fore driven means comprisesa driven sprocket having teeth, the fore drive means comprises a drivesprocket, and the fore drive means driving means comprise teeth; and thetransmission further comprises a chain meshable with the teeth of thedriven sprocket and the drive sprocket.
 4. The transmission recited inclaim 1, further comprising intermediate gear shaft fore and aft bearingmeans for rotatably affixing a fore end and the aft end of theintermediate gear shaft to the housing, respectively.
 5. Thetransmission recited in claim 1, further comprising inner shaft bearingmeans for rotatably affixing a fore end of the inner shaft to thehousing.
 6. The transmission recited in claim 1, wherein the drive shaftfore end is matable with a crank shaft of an engine.
 7. The transmissionrecited in claim 1, further comprising outer shaft bearing means forrotatably affixing an aft end of the outer shaft to the housing.
 8. Thetransmission recited in claim 1, wherein the drive shaft and the innerand the outer shafts are positionable within the housing in generallyvertical alignment.
 9. A propulsion system for driving a pair ofpropellers in counter-rotation, the transmission comprising: an innershaft having means for affixing an outer propeller thereto; a hollowouter shaft having a lumen therethrough and means for affixing an innerpropeller thereto and positionable in surrounding and coaxial relationto the inner shaft; fore driven means affixable to the inner shaft; aftdriven means affixable to the outer shaft, the aft driven means coaxialwith the fore driven means; a drive shaft having an aft end rotatablyaffixable within the housing and a fore end having means for beingrotatably driven; fore drive means affixed for corotation with the driveshaft and having means for driving the fore driven means in a firstdirection; and aft drive means affixed for corotation with the driveshaft and having means for driving the aft driven means in a seconddirection opposite the first direction.
 10. A transmission forconverting an engine drive shaft rotation into coaxial counter-rotationsfor driving a pair of propellers, the transmission comprising: a housinghaving an interior space, a fore side, and an aft side opposed to thefore side, the fore side affixable to an engine bell housing, an openingin the aft side for permitting a pair of coaxial propeller shafts topass therethrough, and an opening in the fore side; fore driven meanspositioned within the housing interior space and affixable to an innershaft for rotating an outer propeller; aft driven means positionedwithin the housing interior space and affixable to a hollow outer shaftcoaxial with the inner shaft for rotating an inner propeller, the aftdriven means coaxial with the fore driven means; a drive shaft having anaft end rotatably affixable within the housing and a fore end extendingthrough the housing fore side opening and having means for beingrotatably driven by an engine; fore drive means affixed for corotationwith the drive shaft and having means for driving the fore driven meansin a first direction; and aft drive means affixed for corotation withthe drive shaft and having means for driving the aft driven means in asecond direction opposite the first direction.
 11. The transmissionrecited in claim 10, further comprising bearing means for rotatablyaffixing: the inner shaft to the housing fore side; the outer shaft tothe housing aft side; and the drive shaft to the housing fore side andaft side.
 12. A propulsion and lubrication system comprising: a hollowinner shaft for rotating an outer propeller having an opening at an endthereof and a lumen therethrough in communication with the hole; ahollow outer shaft coaxial with the inner shaft for rotating an innerpropeller positioned in surrounding and coaxial relation to the innershaft; removable means for sealing the opening in the inner shaft; meansfor driving the inner and outer shaft in counter-rotating relation; andbearing means positionable between the inner and the outer shafts incovering relation to the hole, the bearing means movable relative to theinner shaft; wherein lubricant is sealable within the inner shaft and isdistributable through the hole, between the inner shaft and the bearingmeans, into a space between the inner and the outer shaft, without apump for circulating the lubricant.
 13. The propulsion and lubricationsystem recited in claim 12, wherein the bearing means comprise a helicalgroove in an outer surface thereof.
 14. The propulsion and lubricationsystem recited in claim 12, wherein the inner and the outer shaftcomprise a first material and the bearing means comprises a secondmaterial distinct from the first material.
 15. The propulsion andlubrication system recited in claim 14, wherein the second materialcomprises brass.
 16. The propulsion and lubrication system recited inclaim 12, wherein the bearing means comprises a substantiallycylindrical element slidable over the inner shaft.
 17. The propulsionand lubrication system recited in claim 16, wherein the hole comprises aplurality of sets of holes and the cylindrical element comprises aplurality of cylinders positioned in axially spaced relation along theinner shaft, each of the plurality of cylinders in covering relation toa one of the plurality of sets of holes.
 18. A method for improving anefficiency and lowering a noise output of acounter-rotating-propeller-driven vehicle comprising the steps of:providing a propulsion system comprising: an inner shaft having meansfor affixing an outer propeller thereto; a hollow outer shaft having alumen therethrough and means for affixing an inner propeller thereto andpositionable in surrounding and coaxial relation to the inner shaft;fore driven means affixable to the inner shaft; aft driven meansaffixable to the outer shaft, the aft drive means coaxial with the foredrive means; a drive shaft having an aft end rotatably affixable withinthe housing and a fore end having means for being rotatably driven; foredrive means affixed for corotation with the drive shaft and having meansfor driving the fore driven means in a first direction; and aft drivemeans affixed for corotation with the drive shaft and having means fordriving the aft driven means in a second direction opposite the firstdirection; and utilizing the propulsion system to drive a pair ofpropellers in counter-rotating motion.
 19. A method for making atransmission comprising the steps of: rotatably affixing a fore drivenmeans within a housing and to an inner shaft for rotating an outerpropeller; rotatably affixing an aft driven means within the housing andto a hollow outer shaft coaxial with the inner shaft for rotating aninner propeller, the aft driven means coaxial with the fore output gear;opposedly affixing a fore end and an aft end of a drive shaft forrotation within the housing; mounting a fore drive means on the driveshaft in driving relation to the fore driven means; mounting an aftdrive means on the drive shaft in driving relation to the aft drivenmeans; and providing means for driving the fore and the aft driven meansin counter-rotation.